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The present invention relates generally to the microwave ferrite devices and, more specifically, to the ferrite structures used in those devices which realize non-reciprocal circulation action. Most common ferrite devices are the Y-type circulators. In stripline embodiment, the Y-circulator consists of a conductive central junction situated between a pair of planar ferrite elements. Ferrites are biased externally with the DC magnetic field applied normally to their plane. Two non-ferrous metallic plates attached to the opposite faces of ferrite-junction-ferrite structure provide the electrical ground. The junction is formed of three branches extending by 120 degrees apart from the common central area. In the circulators all three branches are electrically connected to the transmission lines. In the isolators a matched load (usually a 50 Ohm resistor) terminates one of the ports.
Presently, the stripline circulators have two basic setups for the magnetic field application. The first one is a tower-like setup, where the magnets are attached to both sides of the ferrite-junction-ferrite structure. Along with two non-ferrous ground planes this setup includes also a u-shape ferrous shunt clip completing the magnetic loop, and the side cover closing the entire structure. The second setup is a drum-like design, where the magnets (usually three) composing a common plane with ferrite structure are evenly spaced along the structure""s periphery. This setup includes also two ferrous plates (pole pieces) attached to the opposite faces of ferrite-magnets structure. The ferrous plates are required to direct the magnetic flux outgoing from the magnets into the ferrites situated in the central area. The heights of the magnets and ferrite-junction-ferrite stack ideally should be the same to provide a simultaneous contact with both pole pieces.
The existing circulators/isolators incorporate either the soft or hard ferrites, both exhibiting gyrotropic properties in a magnetized state. In order to maintain a magnetized state the soft ferrites should be permanently biased with an external DC magnetic field. The frequency of natural magnetic resonance (resonance at zero external magnetic field) in the soft ferrites equals almost zero. With the available external fields the frequency of magnetic resonance in the soft ferrites can be tuned only to about 20 GHz. Because of that, this class of ferrites is regarded as the low-frequency materials. The high-frequency devices usually incorporate the hard ferrites. Ferrite materials, such as Sr/Ba hexaferrite ceramic, used in those devices, typically exhibit the natural magnetic resonance at the frequencies 40 GHz and above. The hard ferrites are the permanent magnets possessing a considerable residual magnetization. Therefore, once being magnetized they are capable of maintaining the magnetization even without the external magnetic field. In the microwave range the hard ferrites are usually used as self-biased high frequency ferrites.
Typically, the stripline circulators are the narrow-band devices. The bandwidth here is defined as being a difference between the highest and lowest operation frequencies, at which an acceptable insertion loss and required isolation between the corresponding ports are maintained. If the application requires a broadband operation, the circulators should incorporate the wideband matching transformers or composite ferrites (see, for example, U.S. Pat. No. 4,205,281). The composite ferrite is made in such a way that its constituent elements (ferrite puck and rings) are. combined in a radial direction one inside the other, to have the last one encircling the. entire internal portion. The utilization of composite. ferrites in the conventional circulators allows improving the bandwidth performance by providing the circulation at two or more frequencies. This is achieved by selecting the size and magnetization of the external ferrite ring determined as a function of the lowest frequency of the pass band. The second ferrite is selected to have the dimensions and magnetization determined as a function of the second frequency being above the first frequency. The third and additional ferrite elements may be selected using the same approach (see, for example, U.S. Pat. No. 4,496,915). Since a common external magnetizing system is used in this setup, all portions of a composite ferrite are magnetized in the same direction.
In practice, it is difficult to develop a compact and lightweight circulator/isolator operating in a wide frequency range. The application of stronger magnetic fields, the utilization of sophisticated multi-ring ferrite assemblies and complicated matching transformers in order to extend the bandwidth and to increase the operational frequency, requires more space, adds to size, weight and cost. The circulators/isolators are widely used in communication equipment including those used on board of the satellite vehicles, in mobile and hand-held terminals. Therefore, increasing the operational frequency and extending the bandwidth while maintaining a small size and weight, are important goals for the design of circulators/isolators.
For clarity, the present invention will be described in a stripline embodiment only. This, however, does not restrict in any way the scope of present invention, because it can also be implemented with other types of propagation lines, including the microstrip lines, waveguides and quasi-optical beams.
The stripline Y-circulator according to the present invention is comprised of two composite ferrites, central junction, and of at least two ferrous plates. Each composite ferrite represents a monolithic disk-shape body and consists of at least two regions. One of the regions is made from a soft ferrite and another one from a hard ferrite. Both soft and hard ferrite regions have substantially different resonant frequencies. The central junction having basically the Y-shape is situated between the composite ferrites. The ferrous plates are disposed on the external faces of a ferrite-junction-ferrite structure. The hard and soft ferrite regions of the composite ferrites are the parts of a magnetic loop completed via ferrous plates. The direction of magnetization in all hard ferrite regions is the same. The hard and soft ferrite regions are magnetized in the opposite directions. The shape of the central junction is selected to match its impedance to that of the transmission line, thereby minimizing the insertion and reflection losses. The operational bandwidth of a device incorporating this ferrite structure is selected to be between the frequencies of magnetic resonance in the soft and hard ferrites.
Thus, the new ferrite structure according to the present invention is a part of a passive microwave device such as circulator/isolator, where the RF circulation processes are developed. The composite ferrites and ferrous plates in the structure are disposed symmetrically on each side of the junction in parallel relationship with each other. The composite ferrites, each consisting of at least two ferrite portions, the soft and hard ones, have different frequencies of magnetic resonance. Both portions of a ferrite structure exhibit the gyromagnetic properties, while the hard ferrite portion possesses also the permanent magnetic properties. The magnetic flux outgoing from the hard ferrites is trapped within a magnetic loop composed by the ferrous plates and soft ferrites. As a result, the magnetization of the soft ferrites is opposite to the magnetization in the hard ferrites. The operational bandwidth is selected to be between the frequencies of magnetic resonance in the soft and hard ferrite regions.
It is a primary object of the present invention to have a compact and lightweight structure that provides a broadband circulation action, including the frequency domain that is difficult to achieve with the conventional structures (approximately from 20 to 40 GHz).
It is a further object of the present invention to have a structure wherein the areas of magnetic flux creation and confinement would be the region where. the RF circulation process takes place, by this eliminating an extra space for the external magnets.
It is the advantage of the present invention to have a ferrite structure for devices such as circulators/isolators that is easy to produce with the existing technologies, is labor saving and cost efficient.